1. Field of the Invention
This invention relates to two-phase anaerobic digestion of biodegradable feedstocks, such as organic wastes and manure. More particularly, this invention relates to a method and apparatus for two-phase anaerobic digestion of organic wastes, such as manure from large animal production, food processing wastes, as well as energy crops, e.g. grasses, corn etc. for producing hydrogen gas for use in energy production in which the second anaerobic digestion phase is carried out in a bioreactor vessel utilizing hollow semipermeable fibers in which the liquid effluent from the first anaerobic digestion phase is transmitted into the second anaerobic digestion liquid phase and processed by hydrogenogenic cultures present in this liquid phase disposed on the outer surfaces of the hollow semipermeable fibers. The fibers are operated at a mild vacuum (about 15–20 mm H2O). The hydrogen gas generated by the hydrogenogenic cultures is passed through the semipermeable walls of the hollow semipermeable fibers and into the fiber lumen from which it is then removed. The hydrogen gas generated can be used in conjunction with fuel cell technology to generate electricity on the farms and factories producing the wastes, to replace or supplement the needs of the facilities.
The high-organic wastes generated by these farms and factories are now a liability for them and in many cases incur a cost for disposal. In the case of farms and large animal production facilities, the wastes are discharged to a lagoon for anaerobic digestion to proceed until land application is executed. However, lagoons release a number of gases, including greenhouse gases, methane and carbon dioxide to the atmosphere, which increases their negative impact to the environment. This invention converts this liability into an asset.
2. Description of Related Art
The production of methane and other usable gases by anaerobic digestion of various organic wastes is well-known. The organic feed mixture which provides the substrate for anaerobic biodegradation may comprise a wide variety of organic carbon sources, ranging from raw sewage sludge to municipal refuse, or biomass material such as plants and crop wastes. Anaerobic digestion of organic feedstocks generally involves hydrolysis fermentation of organic feedstocks to acidic intermediates by acid forming bacteria and conversion of the acidic intermediates to useful gases, such as methane, by methane-producing organisms. Many digester designs, feedstocks mixtures and additives have been proposed to increase the methane yield from anaerobic digestion and to provide greater conversion efficiency of organic materials to useful products.
Early designs of sewage digesters attempted to biodegrade sewage sludge for the purposes of sludge volume and odor reduction in an unmixed digester, but they were generally unsuccessful because they failed to provide adequate control of solids inventory, and they developed serious problems such as scum buildup, temperature fluctuations, unequal microbial activity and limited contact between the organic material and the bacteria. Most newer anaerobic digesters for biological conversion of biomass and community wastes are continuously stirred tank reactors, which provide complete mixing of the reactor contents. Solids and hydraulic retention times are equal in continuously stirred tank reactors and both hydrolysis fermentation reactions converting organic materials to acidic intermediates and methane-producing reactions converting acidic intermediates to methane and other gases occur throughout the reactors.
Many organic feedstocks have a relatively low suspended solids content, for example less than about 10 percent suspended solids. The high water content of these types of organic feedstocks causes washout of feed solids and microorganisms from continuously stirred tank reactors at high feed loadings due to high dilution rates. Washout of feed solids and microorganisms results in reduced conversion efficiency and unstable digester conditions. Shorter feed solids retention times in the digester, washout of slow-growing methanogenic bacteria and accumulation of inhibitory acidic fermentation products contribute to low conversion efficiency and reduced methane production.
Anaerobic filter-type reactors promote the retention of bacteria in the digester by attaching bacteria to fixed inert materials in the digester. Anaerobic filter-type digesters are limited to primarily liquid feedstocks containing less than about 1 percent solids because they become plugged when solids concentration in the digester increases due to higher solids loading or accumulation of solids over longer periods of operation.
U.S. Pat. No. 4,329,428 teaches production of methane gas in higher yields and at higher rates by thermophilic and mesophilic anaerobic digestion of a mixture of plant material of terrestrial or aquatic origin and organic waste. U.S. Pat. No. 4,424,064 teaches production of methane gas with higher yields and at higher rates by thermophilic or mesophilic anaerobic digestion of aquatic plant material, at least a portion or all of which has been grown in organically polluted water. U.S. Pat. No. 4,316,961 teaches higher yields of methane gas at higher rates by thermophilic or mesophilic anaerobic digestion of plant material and/or organic waste of normally low biodegradability in the presence of an extract of different plant material.
Separated two phase anaerobic digestion processes have been found to enhance the conversion efficiency. See, for example, U.S. Pat. No. 4,318,993. In an acid first phase, the microbial population and operating conditions are selected to promote the conversion of organic carbonaceous matter to volatile fatty acids of low molecular weight. The volatile fatty acids remain solubilized in the liquid portion of the digester contents. The liquid and solid effluent from the acid phase is conveyed to a methane second phase, where methanogenic microorganisms convert the volatile fatty acids to product gas composed primarily of methane and carbon dioxide. Product gas is removed from the methane phase and processed, or scrubbed, to separate the methane component which is drawn off as pipeline gas. U.S. Pat. No. 4,022,665 teaches certain specific operating conditions for a two phase anaerobic digestion process in separated vessels which promotes more efficient conversion of organic material.
The use of hollow semipermeable fibers in connection with the processing and treatment of various liquids is well documented in the prior art. U.S. Pat. No. 4,268,279 teaches microporous hollow fibers with a liquid in the fiber lumen and a fluid outside the fiber allowing gaseous components to transfer through the microporous fiber to the inside or outside of the fiber. U.S. Pat. No. 4,966,699 teaches a hollow fiber membrane fluid processor providing counter current flow of fluid in the fiber lumen and the fluid surrounding the outside of the fibers from one end of the fiber bundle to the other. U.S. Pat. No. 5,198,110 teaches a bundle of permselective hollow fibers having a plurality of filaments extending substantially lengthwise over the length of the exterior of each fiber. U.S. Pat. No. 5,693,230 teaches a hollow fiber contactor and process having forced circulation with entry of fluid to be processed through the open ended lumen of a porous input hollow fiber having its opposite end closed and exit of treated fluid through the open ended lumen of an adjacent or nearby porous output hollow fiber having its opposite end closed. In the contactor, the fluid to be processed passes through the porous wall of an input hollow fiber, contacting a treatment medium and forming a treated fluid which passes through the porous wall of an output hollow fiber and exits the contactor.